Radiated Susceptibility
by Ron Brewer, EMC/ESD Consultant
Some EMC tests are about as exciting as
watching grass grow. However, I must admit that today’s computer
control beats the previous procedures that included highly
manual testing followed by lengthy and often very tedious
rechecking, recording, and analyzing results.
Throughout the years, the electric field (EF)
radiated susceptibility/immunity test remains the most fun EMC
test to run. It’s so easy to produce the required EF at any
desired frequency that we can really concentrate on testing the
EUT.
Performing the RS Test
Radiated susceptibility (RS) testing in its
various forms is mandated by a number of EMC specifications
including MIL-STD-461, MIL-STD-464, and the EU. Those of you who
have read the article titled "EMC Failures Happen" in the
December 2007 issue of EE-Evaluation Engineering know
that passing an EMC test won’t guarantee that the unit will be
immune to EMC problems in its operational environment. But it
helps.
RS testing, illustrated in Figure 1,
requires the EUT to be illuminated by a low-, medium-, or
high-level EF. If it will fit, the EUT is placed in a shielded
enclosure with a layout that represents its normal operational
configuration as closely as possible. The setup includes placing
it on a ground plane made from material representative of the
actual platform.
Figure 1. MIL-STD-461D/E/F RS103 Measurements
* = EF Display and Sensor Required for RS103
A transmit antenna is placed in front of the
EUT’s most susceptible RF pickup area at the separation distance
prescribed by the test specification, typically 1 meter for
military and 3 meters for the EU. The EF is established at the
specified frequencies using a signal generator and an RF power
amplifier to drive the transmit antenna. It may require a suite
of signal generators, power amplifiers, and antennas to cover
the entire frequency range.
The test signal is modulated using a
frequency and waveform that correspond to worst case. In the
event these are unknown, use the modulation called out in the
specification.
The frequency range for the test is slowly
swept at the prescribed EF level or higher. This can be done by
a computer or manually.
Table 1 indicates the MIL-STD-461F
susceptibility scan speeds. At frequencies where the EUT is
susceptible, the scanning is stopped, the EF is reduced to the
susceptibility threshold level, the first level is recorded, and
then the EF is reduced by at least 6 dB. Now the EF level is
increased until the susceptibility condition reappears. This
second level is compared with the first, and the lowest level is
the susceptibility. Performing the test in this way avoids the
problems of hysteresis in the measurement.
Table 1. Susceptibility Scan Speeds
fo = tuned frequency
Most test labs execute susceptibility tests
manually because of the randomness of susceptibility
occurrences. It’s difficult enough to establish the
susceptibility criteria. But then these criteria must be
monitored and provided as feedback to a computer to tell it to
stop when susceptibility occurs, automatically adjust RF levels
to 6 dB below the susceptibility point, and perform a retest.
Humans do a much better job of correlating
what may appear as random, nonrelated incidents. It’s not
uncommon for an absolutely unexpected, unplanned susceptibility
condition to occur.
RF Test Equipment
The RS test requires equipment to create the
EF and monitor it just to ensure that the EF is there. Following
is a list of the equipment that would be used to do a
MIL-STD-461E/F RS test:
Transmitters to Create the EF
• LISNs: Used to standardize the power input
RF impedance.
• RF Signal Generators: Any standard
generator with modulation that covers the frequency range.
• Modulation Generator: Used with the
standard generator to provide the required modulation.
• Power Amplifier (PA): An RF signal booster
used with the standard generator because no generator has enough
output to produce the required EF.
• Transmit Antennas: Produces the EF. The EF
required in conjunction with the gain of the transmit antenna
determines the size of the PA.
• Directional Coupler: Because of antenna
impedance mismatch variations, the directional coupler is
required to determine forward power when precalibrating the EF.
• Power Meter: Used with the directional
coupler.
Receivers to Ensure the EUT Is Exposed to the
Correct EF
• EF Sensors: 10 kHz to 1 GHz. Used at the
EUT to determine incident EF strength. Sensors that cover the
1-GHz to 18-GHz range also are available.
• Receive Antennas: Used at the EUT to
determine incident EF strength in place of EF sensors; 1-GHz to
10-GHz double ridge horns and 10-GHz to 40-GHz antennas as
approved by the procuring activity.
• Attenuator: Used to protect the measurement
receiver and reduce errors from antenna VSWR.
• Measurement Receiver: Used with the receive
antennas. One or more may be required to cover a test frequency
range; could also be a spectrum analyzer.
• Data Recording Device: Connects directly to
receiver output or a computer used to control the receiver.
From an EMC perspective, there’s nothing
unusual about a test setup that uses signal generators, RF power
amplifiers, and antennas until it’s time to perform such a test.
Then try to find that equipment. All the equipment is readily
available except for large broadband RF power amplifiers and
antennas.
Since the antenna performance determines the
amplifier power requirement, it’s necessary to know the
worst-case antenna gain and how far we need to squirt the RF to
size the amplifier. Figure 2 shows how to calculate the
power amplifier requirements based on antenna characteristics.
Figure 2. RF Power for Required EF
The same set of amplifiers can be used with a
wide assortment of transmit antennas. And there is a wide
assortment, each with very different characteristics. It would
be great if one antenna could be used to generate the RF field
across the entire frequency range, but dimensional restrictions
and antenna Q limit the maximum antenna bandwidth to about a
decade. Table 2 shows the most popular antenna types used
in the different frequency ranges.
Table 2. Antenna Types vs. Frequency
Generating high-amplitude EF strengths in the
10-kHz to 30-MHz frequency range is difficult because antenna
dimensions are very small with respect to a half wavelength,
making the antenna efficiency very poor. As a result, there are
some antenna alternatives such as the GTEM cell and parallel
plate/triline used when testing smaller EUTs. For larger EUTs,
the size of the line limits the usable upper frequency.
There always has been a struggle regarding
the sizes of the EUT, the shielded enclosure, and the antennas.
To minimize distortion and antenna loading, when an antenna is
used in a shielded enclosure, the ends should be kept away from
the wall by at least 0.5 meter. For small EUTs, the size of the
antenna determines the enclosure size.
In the early days of RFI/EMI/EMC testing, RS
tests were performed by feeding the 100,000-µV modulated output
of a standard signal generator into a 41-inch monopole, tuned
dipole, or horn antenna. E-fields weren’t monitored.
MIL-STD-826 (1964), the first attempt at a
tri-service standard and the basis of a number of procedures in
MIL-STD-461, changed all that. Then, RS field strengths were
monitored by antennas placed to the side or behind the transmit
antennas.
Now for MIL-STD-461 measurements, we’ve
shrunk the antennas, grouped three together aligned along
the X-Y-Z axes, added amplification
to make up for their inefficiency, called them EF probes, and
placed them on or in close proximity to the EUT. To minimize EF
probe susceptibility and field distortion, most utilize
fiber-optic interfaces.
The EU EMC tests use an alternative approach
in which the EF is precalibrated. Figure 3 shows the EU
16-point EF uniformity requirements.
Figure 3. EU EF Uniformity +6/-0 dB
Defining Susceptibility
We want to determine if the EUT will operate
properly in an adverse RF environment. This can be defined by
duplication of previously measured RF environmental levels or
compliance with an EMC specification. Susceptibility to radiated
EM energy primarily is due to RF pickup on wires and cables and generally results in
malfunctions or degradation of performance. The latter often can be
tolerated, but malfunctions cannot.
The problem of establishing pass/fail
criteria for susceptibility is determining how much degradation
is tolerable before we conclude that the EUT is not working
properly. Beware of any specification that states that the
characteristics of the EUT during the susceptibility tests
cannot change from those measured in the laboratory sans RF.
Four characteristics greatly influence the
susceptibility of the EUT: frequency, amplitude, spatial
relationships, and timing (FAST). They often are used as a
culling approach to analyze EMC problems.
Frequency
For an RF device, in-band susceptibility
generally stems from the culprit frequency or its harmonics
coupling at the victim’s tuned frequency, harmonic, or IF. Out-of-band or
non-RF device susceptibility generally results from the culprit
frequency coupling into a circuit through an RF response window
created by wire, cable, or parasitic resonances.
Cable resonance is one of the most often
occurring problems so failures frequently occur in the 30-MHz to
300-MHz range. Because the response frequencies are unknown, the
entire RF spectrum must be scanned during an RS test. Signals
must be modulated to determine if the system is susceptible to
audio rectification.
Amplitude
The interfering signal adds to the EUT
internal noise. If the amplitude
of the interfering signal being coupled into the EUT is at the
same level as the intended signal, most likely the system will
malfunction. Consequently, the amplitude of the interfering
source energy level determines the susceptibility.
Spatial Relationships
If the EUT is susceptible along a particular
axis, then the field-generating antenna should be aligned with
this axis during the test. It’s not a big problem when the test
is performed in an ordinary metal box enclosure, but it is in an
anechoic enclosure.
The EU requirements handle this problem by
rotating the EUT. Military tests generally probe the unit for
the worse emission locations and assume that will correspond
with the worst susceptibility locations.
Timing
Susceptibility occurs only when both the
culprit and victim are ON. The ease of determining
susceptibility then depends on whether the simultaneous
operation of the culprit and the victim make their timing
relationship appear random, periodic, or continuous.
It’s possible to use FAST analysis to
understand what circuits, subsystems, or equipment are likely to
respond to the susceptibility signal and under what
circumstances this may occur. As an example, the operational
transfer function of an analog circuit is continuous. A slight
change in the input results in a large change in the output.
Accordingly, analog circuits respond to much lower RF
susceptibility signals than digital equipment, typically on the
order of 40 dB. Once the analog signal is contaminated, there is
no way to clean it up.
Digital circuits, on the other hand,
require significantly higher susceptibility signal levels before
malfunctions occur. These malfunctions generally are the result
of a change of state of the logic devices and take place
suddenly with a corresponding loss of data. Prior to this point,
digital devices will operate properly.
During susceptibility testing, there
is a requirement to place the EUT in its most susceptible
operating mode.
The difference in the behavior of analog and digital circuits
really complicates determining what mode is the
most susceptible.
The number of modes multiplies the test time
accordingly. If the EUT has one mode, the test is performed
once. If it has 10 modes, the test is performed 10 times.
Fortunately, MIL-STD-461 only requires a sufficient number of
modes to ensure that all circuitry is evaluated.
Since real-time monitoring is necessary,
tests usually are performed using built-in test equipment
(BITE). This procedure is augmented by special test software,
custom interface circuits, and the creative use of fiber-optic
interconnects, isolation transformers, closed-circuit TV,
acoustic couplers, telescopes, shotgun microphones, canary
circuits, and any other thing necessary to monitor the operation
of the EUT.
Care must be exercised to ensure that these
equipment modifications, special circuits, and software do not
change the susceptibility of the EUT. Honesty is important. It’s
generally up to the test director to be sure that the test is
being performed properly.
Test Conditions
Typically, there are three distinct areas
used for EMC testing: the measurement equipment area, the EUT
test environment area, and the exercise-simulation equipment
area. The EUT test environment area for RS generally is a
shielded enclosure.
The test configuration should isolate these
three areas. In the simplest arrangement, the shielded enclosure
is placed between the other two areas. In a more extensive
setup, three or four shielded enclosures are configured so each
area is contained within its own shield (Figure 4).
Figure 4. Suggested Test Area Layout
A number of groups, such as the FAA, the FCC,
the IT department, and our neighbors, would be upset if we just
started generating RF energy without regard to the environment.
As a result, most RS tests are performed in a shielded
enclosure.
There are three primary types of shielded
enclosures: standard metal boxes, anechoic enclosures, and
reverberation chambers. The standard metal box enclosure more
closely represents the actual operational environment,
especially for military equipment. Most military aircraft,
ships, tanks, Humvees, communications shelters, Quonset huts,
and field desks are made of metal. The military even uses
shielded tents made from nickel-plated nylon.
However, there are problems associated with
testing in a standard metal box: enclosure resonance,
reflections from the walls, and antenna loading. The biggest
problem is the metal box itself, which is a resonant cavity. At
resonance, the EF levels can be increased as much as 35 dB to 40
dB. The field distribution is not uniform, and since the Q is
very high, it’s possible to mistake a field-intensity increase
at room resonance as EUT susceptibility. The same is true
regarding narrowband emissions from the EUT.
We can fight the problem, or we can work with
it. If you want to fight the problem, resonance can be checked
and the effects significantly reduced by detuning the cavity.
This can be done by opening the door or using portable anechoic
panels.
Panels work best. These typically are 8.5 ft
high and made from a 4 ft x 8 ft sheet of ¾ inch plywood with
absorber material on one side and aluminum foil on the other
side. These roll-around panels can be used in absorber-enhanced
enclosures.
Unlike the EU EMC requirements, MIL-STD-461
does not require a semi- or fully anechoic enclosure, only that
RF absorber material be placed above, behind, and on both sides
of the EUT from the ground plane to the ceiling with additional
absorber located behind the test antenna from the floor to the
ceiling. The absorber material must provide at least 6-dB
attenuation from 80 MHz to 250 MHz and 10 dB or more above 250
MHz.
The requirement to place the EUT no closer
than 30 cm to the absorber material, combined with absorber
material that could be 24 to 30 inches thick, means that the
material significantly reduces the working volume inside the
shielded enclosure. The enclosure working volume is determined
by the size of the EUT or the size of the antennas, whichever is
larger.
A reverberation chamber works with the
enclosure resonances and reflections. It has a useable volume
approximately 50% of the total and can be smaller than an
absorber-lined enclosure. A rotating metal reflector sweeps the
maximum field strength produced by resonance and reflections
throughout the enclosure to assure that the EUT has been
adequately illuminated by the EF.
A shielded enclosure is a test equipment
item. It has or should have a model number and a serial number.
It is not a room. It is a large metal enclosure with a power
line and other types of filters that contains or excludes RF
energy. It should be placed on a calibration schedule just like
all other test equipment.
In the case of an enclosure, make sure that
the internal ambient is 6 dB or more below the specification
limit. Data from the calibration is kept in a file. Lastly,
unless the test sample is huge or it must do something strange
during its operation, open area test sites (OATS) normally are
not used for performing RS tests.
RF Safety/RADHAZ
MIL-STD-461D/E/F calls out 200 V/m. With
20-dB to 40-dB resonant gain inside an enclosure, the E-field
strength in a shielded enclosure where a 200-V/m field is being
generated could be 2,000 V/m to 20,000 V/m at some locations
within the enclosure.
Beware that high-level RF fields are
hazardous to your health. There is plenty of information that
discusses how exposure of this type can cause the formation of
cataracts or thermal tissue damage.
The FCC OET Bulletin 65 covers RF
hazards and provides the equations for calculating how long you
can be exposed before a hazard exists. These equations are based
on thermal heating, but they do consider antenna gain,
modulation type, and antenna separation.
Not a lot of information cites actual health
hazards for lower-level RF fields because of a lack of concrete
proof that such dangers exist. The premise is that observable
biological effects do not necessarily mean that there is a
biological hazard. But of course, it doesn’t rule out that
possibility either.
This disagreement about what constitutes an
RF hazard level is reflected in the differences between the
Russian RADHAZ and U.S. safety levels. The Russian levels are
based on the field strength at which there are observable
effects; the U.S. levels are based on the field strength where
thermal damage occurs. Consequently, the Russian maximum
field-strength levels are much, much lower than in the United
States.
If observable biological effects are
occurring, then there is likely to
be an RF hazard, even if we haven’t yet determined what the
hazard is. Accordingly, precautions should be taken. Call it
prudent avoidance, but why take the chance?
Measurement Error and Uncertainty
During an RS test, an attempt is being made
to control the RF environment. But testing problems are
compounded because of variations in the characteristics of the
EUT, measurement equipment, and test setup/facility. These
variations can result in large differences in EMC measured data,
often as much as 40 dB.
It is not possible to make measurements
without the measurement process/equipment disturbing the data.
Some of the errors will be random, others systematic. If the
errors are reasonably independent, a calculation of the total
uncertainty can be made by combining their standard deviations.
This uncertainty value will tell how much
potential error there is in the measurement. However, it does
not tell us anything at all about the probability of
susceptibility capture. That is, what is the probability that a
complex system will be in an internal state where it is
susceptible concurrent with the presence of a signal with
frequency and modulation characteristics that would cause
susceptibility?
If you are interested in learning more about
the statistical nature of EMC measurement uncertainty, check out
IEC CISPR 16-4. It deals only with the random and systematic
errors associated with making measurements on a steady-state
device to an EMC standard. It does not address how well the
standard’s test environment mimics the actual operational
environment.
About the Author
Ron Brewer currently is a senior EMC/RF
engineering analyst with Analex at the NASA Kennedy Space
Center. The NARTE-certified EMC/ESD engineer has worked
full-time in the EMC field for more than 30 years. Mr. Brewer
was named Distinguished Lecturer by the IEEE EMC Society and has
taught more than 385 EMC technical short-courses in 29 countries
and published numerous papers on EMC/ESD and shielding design.
He completed undergraduate and graduate work in engineering
science and physics at the University of Michigan. e-mail:
ronbrewer@ieee.org
FOR MORE INFORMATION
on FCC OET Bulletin 65
Click here
on RF/ELF handbook for health professionals
Click here